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Key component of Mercury's terrain, covering about 40% of its surface, located between later craters and evenly distributed around the planet. Likely volcanic in origin, it is the oldest surface known on the planet, predating the heavily cratered terrain. Appears to have covered up some older craters and younger than the Caloris Basin. Unlike volcanic plains of the Moon, it is the same base color as the older inter-crater plains. Characterised by rupes which criss-cross the plains, perhaps due to planetary compression.